Flutter Validator String 需要 Null Aware 但编译器警告不需要

Question

我有一个 TextFormField：

TextFormField(
  textAlign:
      TextAlign.center,
  autovalidateMode:
      AutovalidateMode
          .onUserInteraction,
  onChanged: (value) {},
  controller:
      firstNameTextInputController,
  validator: (input) =>                         //////////////////////////// HERE
      (input!.length <
                  51 &&
              input!.length >
                  1)
          ? null
          : "Invalid First Name",                                    ////// End
  style: TextStyle(
    color: HexColor
        .fromHex(
            "#000000"),
  ),
  decoration:
      InputDecoration(
    border: InputBorder
        .none,
    filled: true,
    hintStyle: Theme.of(
            context)
        .textTheme
        .subtitle1!
        .merge(TextStyle(
            color: HexColor
                .fromHex(
                    "#707070"))),
    hintText:
        "*Enter First Name",
    fillColor: HexColor
        .fromHex(
            "#ffffff"),
    focusedBorder:
        OutlineInputBorder(
      borderSide: BorderSide(
          color: HexColor
              .fromHex(
                  "#707070"),
          width: 5),
      borderRadius:
          BorderRadius
              .circular(
                  5),
    ),
  ),
))),

这会发出警告：

warning: The '!' will have no effect because the receiver can't be null. (unnecessary_non_null_assertion at [athelite] lib\Pages\PlayerEditPageDefaultState.dart:427)

所以我删除了感叹号，然后它变成了一个错误：

error: The property 'length' can't be unconditionally accessed because the receiver can be 'null'. (unchecked_use_of_nullable_value at [athelite] lib\Pages\PlayerEditPageDefaultState.dart:425)

编译器一点都不讨喜！使用 Flutter 2.0 执行此操作的正确方法是什么？

Answer 1

第一个警告列在this documentation

The analyzer produces this diagnostic when the operand of the ! operator can’t be null.

这是因为您在同一 && 运算符的两侧使用运算符 !:

...
(input!.length < 51 && input!.length > 1)
...

如果满足第一个条件，则第二个条件将对操作数 input 的非空值进行操作，从而产生上述警告。

要关闭它，只需删除右侧的 !：

(input!.length < 51 && input.length > 1)

Flutter Validator String 需要 Null Aware 但编译器警告不需要

Flutter Validator String Requires Null Aware But Compiler Warns Not Needed

validation

flutter

dart-null-safety

flutter2.0